When it is desired to join silicone rubber to various substrates, one common practice is to previously apply primer compositions. A number of primer compositions are known. For example, JP-B 2107/1986 corresponding to U.S. Pat. No. 4,332,844 discloses a primer composition comprising an organic silicon compound having an alkoxy group, an organic titanium compound, and an organic silicon compound having an SiH group. JP-A 25615/1994 corresponding to U.S. Pat. No. 5,326,844 discloses a primer composition comprising an organic silicon compound having a group represented by the formula: -CR.sup.2 R.sup.3 (CH.sub.2).sub.n COOR.sup.1. JP-B 4866/1986 discloses a primer composition comprising an organopolysiloxane raw rubber, organopolysiloxane resin, organoalkoxysilane or siloxane, organic titanate, and organohydrogensiloxane. Further, JP-B 48904/1988 discloses a primer composition comprising an organopolysiloxane raw rubber, organopolysiloxane resin, organoalkoxysilane or siloxane, organohydrogensiloxane, platinum series catalyst, organic titanate, and organic peroxide.
As dry plain paper copiers (PPC) and laser beam printers (LBP) are reduced in weight and size, there is an increasing demand for thermoset silicone rubber having a low hardness as typified by a JIS A scale hardness of 20 or lower. With conventional primer compositions as mentioned above, most thermoset silicone rubbers having a low hardness cannot be bonded to various substrates. There is a desire to have a primer composition which can bond such low hardness thermoset silicone rubber to substrates.